Epidemiology

Hematogenous osteomyelitis
- More common in children and adolescents
- Incidence is increasing in adults, driven by a rise in vertebral osteomyelitis
Exogenous osteomyelitis: more common in adults

Etiology

Hematogenous osteomyelitis (endogenous osteomyelitis): caused by hematogenous dissemination of a pathogen
Exogenous osteomyelitis: caused by a spread of bacteria (typically multiple pathogens) from the surrounding environment
- Posttraumatic: infection following deep injury (penetrating injury, open fractures, severe soft tissue injury)
- Contiguous: spread of infection from adjacent tissue
  - Secondary to infected foot ulcer in patients with diabetes
  - Iatrogenic (e.g., postoperative infection of a prosthetic joint implant)

Overall: Staphylococcus aureus
Sexually active young adults: Neisseria gonorrhoeae (presents as septic arthritis, tenosynovitis, dermatitis).
Sickle cell disease: Salmonella species, S. aureus.
IV drug users (IVDU): Pseudomonas aeruginosa, Candida, S. aureus (often involves vertebral spine, clavicle).
Prosthetic joint/hardware: Staphylococcus epidermidis.
Puncture wound through shoe: Pseudomonas aeruginosa.
Cat/dog bites: Pasteurella multocida.
Diabetic foot ulcer (polymicrobial): S. aureus, Gram-negatives, anaerobes.
Vertebral osteomyelitis (Pott disease): Mycobacterium tuberculosis.

X-ray: low sensitivity and specificity for osteomyelitis
- Indication: initial evaluation as can also exclude differential diagnoses of osteomyelitis
- Characteristic findings
  - Acute osteomyelitis: typically no pathological findings
  - Subacute/chronic osteomyelitis: bone destruction, sequestrum formation, periosteal reactions
MRI with and without IV gadolinium: most sensitive study
- Indications
  - Suspected acute osteomyelitis (evidence of inflammation can be seen ≤ 5 days after onset of infection)
- Characteristic findings
  - Acute/subacute osteomyelitis: cortical destruction, bone marrow inflammation, soft-tissue involvement
    - Focal hyperperfusion and increased radiotracer uptake within the affected region.
  - Chronic osteomyelitis: fibrotic scarring of the marrow